(1) Photonic Crystals
Recently, photonic crystals, which are optical functional materials having a cyclic distribution of refractive index, have been gaining attention. A photonic crystal is characterized in that its cyclic distribution of refractive index forms a band structure with respect to the energy of light or electromagnetic waves, thereby creating an energy region (called the photonic bandgap) that disallows the propagation of light or electromagnetic waves. It should be noted that this specification uses the term “light” or “optical” as inclusive of electromagnetic waves.
Introduction of an appropriate defect into the photonic crystal will create an energy level (called the defect level) within the photonic bandgap. This allows only a specific wavelength of light having an energy corresponding to the defect level to exist within the wavelength (or frequency) range corresponding to the energy levels included in the photonic bandgap. Forming a linear defect will provide a waveguide, and forming a point-like defect will provide an optical resonator. The resonance wavelength, i.e. the wavelength of light that resonates at the point-like defect, depends on the shape and refractive index of the defect.
Using such resonators and waveguides, research has been conducted to manufacture various types of optical devices. For example, the resonator can be located in proximity to the waveguide to create a multiplexer/demultiplexer capable of functioning as the following two devices: an optical demultiplexer for extracting a ray of light whose wavelength equals the resonance wavelength of the resonator from rays of light having different wavelengths and propagating through the waveguide, and for emitting the extracted light through the resonator to the outside; and an optical multiplexer for trapping a ray of light having the resonance wavelength of the resonator from the outside, and for introducing the trapped light through the resonator into the waveguide. Such a multiplexer/demultiplexer can be used, for example, in the field of optical communications for wavelength division multiplexing communication in which rays of light having different wavelengths are propagated through a single waveguide, with each ray of light carrying a different signal.
Photonic crystals can be created from one-dimensional, two-dimensional or three-dimensional crystal, of which two-dimensional crystals are advantageous in that they are relatively easy to manufacture. For example, Patent Document 1 discloses a two-dimensional photonic crystal and an optical multiplexer/demultiplexer, each of which includes a two-dimensional photonic crystal consisting of a plate (or slab) with a high refractive index and including a cyclic array of a material whose refractive index is lower than that of the material of the plate, where a waveguide is formed by creating a linear defect of the cyclic array, and a point-like defect (or a resonator) that disorders the cyclic array is formed in proximity to the waveguide. In the present patent application, a waveguide formed within the two-dimensional photonic crystal as described above is called the “in-crystal waveguide.”
[Patent Document 1] Japanese Unexamined Patent Publication No. 2001-272555 (paragraphs 0019-0032; FIG. 1)
In the most typical construction of the two-dimensional photonic crystal, the low refractive index areas cyclically arranged within the slab made of a high refractive index material are made of air (namely, they are holes); this construction yields the largest possible difference in refractive index and is easy to manufacture.
In the two-dimensional photonic crystal described in Patent Document 1, the slab is in contact with air on its upper and lower sides. Since, as explained previously, the difference in the refractive index between the slab and air is large, most of light propagating through the in-crystal waveguide is confined within the slab due to the total reflection, so that a high level of propagating efficiency is obtained.
(2) Relationship Between the Substrate of the Two-Dimensional Photonic Crystal and a Resonator (Point-Like Defect)
In general, the slab is relatively weak in its thickness direction because it should be very thin, as known from Patent Document 1 disclosing an embodiment where the thickness is approximately 0.25 μm. Particularly, two-dimensional photonic crystals created by forming a number of holes in the slab are very fragile in the thickness direction. Lack of the strength in the thickness direction will cause some problems, such as a low level of product yield.
A possible method for strengthening the two-dimensional photonic crystal is to use a substrate (or clad) to support the two-dimensional photonic crystal (this structure is called the “substrate-attached photonic crystal” hereinafter). In this case, the upper surface of the crystal is in contact with air while the lower surface is in contact with the substrate.
However, the characteristics of a point-like defect formed as a resonator in a substrate-attached photonic crystal will be poorer than that of a point-like defect formed in a “substrate-less photonic crystal,” i.e. a two-dimensional photonic crystal in which the slab is in contact with air on both the upper and lower surfaces. FIGS. 1(a) and 1(b) show experimentally measured spectrums of the resonance wavelength of resonators consisting of point-like defects formed in a substrate-less photonic crystal (FIG. 1(a)) and a substrate-attached photonic crystal (FIG. 1(b)) having the same shape except for the presence of the substrate. The half-width of the spectrum in FIG. 1(b) is larger than that shown in FIG. 1(a). Therefore, the substrate-attached photonic crystal is less advantageous than the substrate-less photonic crystal with respect to the wavelength resolution of the resonator. Also, the Q-value, which indicates the performance of resonators, is Q=3,000 for FIG. 1(a) and Q=250 for FIG. 1(b), which means that a larger amount of light energy is lost from the resonator of the substrate-attached photonic crystal.
(3) Relationship Between the Substrate and a Wire Waveguide
The present inventors have been studying two-dimensional photonic crystals having a wire waveguide; it consists of a two-dimensional photonic crystal with a wire waveguide connected to it in order to introduce light from the outside into the in-crystal waveguide of the two-dimensional photonic crystal or extract light from the in-crystal waveguide to the outside. FIG. 2 shows an example. The two-dimensional photonic crystal 10 is formed by cyclically arranging holes 12 in the slab 11, and an in-crystal waveguide 13 is formed by omitting one line of the holes 12. The wire waveguide 14 is connected on an extension of the in-crystal waveguide. By making the wire waveguide from the same material as that of the slab, it is possible to integrate the two-dimensional photonic crystal with the wire waveguide.
The present inventors have calculated the relationship between the frequencies and the wave numbers of light propagating through the in-crystal waveguide and the wire waveguide (i.e. the guided mode). The result demonstrated that, if the entire surface of the wire waveguide is in contact with air, two different modes exist within the wire waveguide. In other words, two modes of light are simultaneously guided by the same waveguide. This means that the guided mode is the multimode in which two wave numbers of light can exist for a single frequency. The two modes of light propagate at different speeds. Such a multimode propagation may cause some problems in optical communications.
In contrast, if, as shown in FIG. 3, a clad member 15 made of a material having a refractive index lower than that of the wire waveguide and higher than that of air is located on one side of the wire waveguide 14, then the guided mode of the wire waveguide will be the single mode (as shown in FIG. 4), which allows only a single wave number of light for one frequency, so that the above-described problem never arises.
In summary, for a two-dimensional photonic crystal having a wire waveguide, it is preferable for the slab of the two-dimensional photonic crystal to be in contact with air (and, accordingly, out of contact with the clad member) on both the upper and lower sides, whereas the wire waveguide should be preferably in contact with the clad member.
Patent Document 1 discloses a method for manufacturing a substrate-less photonic crystal (having no wire waveguide). The method uses a substrate consisting of an InP or Si layer (called the “slab layer” hereinafter) located on an InGaAsP or SiO2 layer (called the “clad layer” hereinafter). First, a two-dimensional photonic crystal is created by cyclically forming holes penetrating through the slab layer. In this process, a point-like defect and an in-crystal waveguide are also created by appropriately setting the diameter or arrangement of the holes. Next, an etchant is introduced into the completed holes to etch the clad layer located under the holes. By maintaining the etching process for a specific period of time or longer, it is possible to further etch the clad layer located between the holes and thereby form a single space (i.e. the air-bridge space) under the entire area of the two-dimensional photonic crystal where the holes are located. The two-dimensional photonic crystal thus manufactured has a bridge-like structure (i.e. the air-bridge structure) including the slab of the two-dimensional photonic crystal formed like a bridge over the air-bridge space.
One of the most natural methods for integrally manufacturing the two-dimensional photonic crystal and the wire waveguide, is to simultaneously form the patterns of the two-dimensional photonic crystal and the wire waveguide on the slab layer and then create the two-dimensional photonic crystal having the wire waveguide by an etching process or other techniques at one time. However, if the etching is performed to create an air-bridge space under the two-dimensional photonic crystal section of the two-dimensional photonic crystal having the wire waveguide manufactured as described above, the problem arises that the etchant infiltrates into the clad layer through the vicinity of the wire waveguide and etches the clad layer under the wire waveguide as well as under the two-dimensional photonic crystal. As a result, the wire waveguide will be out of contact with the clad layer, thus causing the multimode propagation. Furthermore, due to the lack of strength, the wire waveguide may be broken. Putting a protective mask around the wire waveguide cannot perfectly prevent the infiltration of the etchant through the gap between the wire waveguide and the mask; a portion of the clad layer under the wire waveguide will be etched.
In addition, a discrepancy may occur between the wire waveguide and the extension line of the in-crystal waveguide, which will deteriorate the light-guiding efficiency between the in-crystal waveguide and the wire waveguide. Therefore, while manufacturing a two-dimensional photonic crystal having a wire waveguide, it is necessary for the in-crystal waveguide and the wire waveguide to be least dislocated from the predetermined position.